1. Field of the Invention
The present invention relates to an improved process for treating organic sewage which includes organic pollutants, and more particularly to a sewage treatment process for purifying sewage such as domestic and industrial wastewater by using activated sludge under aerobic conditions.
2. Description of the Prior Art
Various processes have been proposed heretofore for treating organic sewage. One known process is carried out in the following manner using a sewage treatment system such as shown in FIG. 6.
Specifically, organic sewage containing microorganisms is introduced into an aeration tank 51 through an influent pipe 52, and air is supplied to the aeration tank 51 from nozzles 54 of a diffusion pipe 53 connected with a blower (not shown) so as to stir the sewage in the aeration tank 51.
Microorganisms proliferate under aerobic conditions to produce activated sludge, and floc having good adsorptivity is generated which is useful for purifying the sewage.
The sewage including floc is introduced by a guide pipe 55 through a baffle tube 56 into a settling tank 57 where the sewage is separated into clarified liquid 58 and floc sediment 59 based on the difference in specific gravity.
The clarified liquid 58 overflows a water receiving flange 60 in the form of a saucer and is discharged to a predetermined place. The floc sediment 59 is wholly or partly returned to the aeration tank 51 through a recovery pipe 61 provided with a pump 62 to be utilized in the subsequent sewage treatment, or it may be discarded.
Such a process using activated sludge, however, has a limit in its treating capacity.
More specifically, the load of organic pollutants given by biochemical oxygen demand (BOD) per volume per time is generally required to be within the range of 0.3 to 1.0 kg BOD/(m.sup.3)(day). Application of an excess load prevents formation of floc having a good coagulation property and develops a bulking phenomenon. If such bulking occurs, floc fails to settle, so that floc may not be separated in the settling tank 57, and consequently the sewage may not be purified.
Therefore, in the conventional treatment process using activated sludge, the treatment system (particularly the aeration tank) tends to become large in size in order to set the organic load within the safe range to avoid the bulking even when the sewage contains a high concentration of organic pollutants. This has caused increases in cost and space of installation of the treatment system.